An ongoing goal in the filtration industry is to perform filtration continuously for sustained periods, even when processing fluids with high amounts of solids or colloidal materials. A wide variety of filter media designs and configurations have been used in attempts to provide a continuous filtration process. This goal has led to several known techniques for continuously inhibiting the buildup of scale, solids cake or films which tend to deposit on and block passage of desired fluid flow through associated filter media. In some cases, these techniques are used intermittently, to perform what is called cyclic cleaning of filter media surfaces, usually when an associated filtration process has been suspended for such cleaning.
Filtration systems generally require periodic removal of clogged filter media or cleaning of filter media to remove particulate matter, solids and/or colloidal matter. Such materials often build up on upstream surfaces of filter media and reduce the rate permeate or clarified fluids may flow through the filter media. Examples include buildup of mineral scale, bridged solids cake or biological films. Intermittently stopping a filtration process to manually or chemically clean upstream surfaces of filter media or to backwash clarified fluid through associated filter media is generally inefficient, labor-intensive, and expensive.
In order to achieve sufficient surface area to serve in commercial scale filtration applications, multiple layers of filter media such as flat sheet membranes are often collected together within a single filtration device. Combining a plurality of sheet membranes may include arranging them in parallel stacks, mounted either horizontally or vertically, or, winding a relatively long single sheet into a spiral configuration.
Various batch cleaning and manual cleaning techniques have been used, such as backwashing, chemical washing or hand scrubbing of filter media. Other methods for inhibiting or alleviating scaling, caking and/or filming of filter media include application of relatively violent vibration of an entire filtration device parallel to the planes of a plurality of stacked filter media and directing air or other gaseous bubbles under pressure parallel with associated filter media.
U.S. Pat. Nos. 4,872,988; 4,952,317; 5,014,564; 5,725,767 and 6,322,698 teach relatively violent reciprocating, torsional vibration of an entire filtration devices parallel to the planes of associated stacked membranes. The patents teach shaking enclosing vessels, stacked filter leaves or plate frame filters along with associated plumbing and connecting devices, and the contained process fluid. Relatively high construction costs may be required to build structures that can withstand these constant reciprocating motions and high amounts of energy often required to generate such motion to provide commercially viable amounts of upstream membrane cleaning, for applications of sufficient value to justify the costs.
Another method used to inhibit membrane clogging by caking, scaling or filming, is the use of air bubbling. U.S. Pat. No. 6,287,467 teaches cleaning parallel mounted flat leaf elements via air bubbling. The associated leaf filter elements generally require maintenance of uniform, structurally braced spacing between each filter leaf element to provide access for air bubbles to all membrane surfaces. The rigidly held membrane surfaces may provide a highly stable platform on which solids cake may build up which the air bubbles can no longer remove such that manual cleaning may be required.
Filtration systems have previously included membranes and other types of filter media with rigid and solid reinforcing structures disposed generally parallel to, and in some cases within, a leaf type filter element or a plate frame filter element. Vibratory techniques such as ultrasonic excitation have been used for sensing membrane conditions, or applied to a single membrane surface, such as in small-scale laboratory explorations.